Method for fabricating a cavity structure, for fabricating a cavity structure for a semiconductor structure and a semiconductor microphone fabricated by the same

ABSTRACT

Embodiments show a method for fabricating a cavity structure, a semiconductor structure, a cavity structure for a semiconductor device and a semiconductor microphone fabricated by the same. In some embodiments the method for fabricating a cavity structure comprises providing a first layer, depositing a carbon layer on the first layer, covering at least partially the carbon layer with a second layer to define the cavity structure, removing by means of dry etching the carbon layer between the first and second layer so that the cavity structure is formed.

This is a divisional application of U.S. application Ser. No.12/979,104, which was filed on Dec. 27, 2010, and is hereby incorporatedherein by reference.

TECHNICAL FIELD

Embodiments of the invention concern a method for fabricating a cavitystructure, e.g. for forming a cavity structure for a semiconductordevice. Several embodiments concern the fabrication of a semiconductorstructure and the fabrication of a cavity structure for a semiconductormicrophone. Furthermore embodiments relate to a semiconductor microphonewhich is fabricated using the methods described herein.

The fabrication of a cavity structure, a cavitation or a hollowstructure may be an important part during the manufacturing process of adevice or device structure. Such a device or device structure may be,for example, used in the field of mechanics, electronics, optics or inmedicine. The fabrication of such a cavity structure may be needed, forexample, for the formation of micromechanical, micro-electromechanical(MEMS), micro-optical or micro-electronic devices. This means, themethod for fabricating a cavity structure may be used in different kindof technical fields. The cavity structure may be manufactured using asacrificial layer which is afterwards removed in an advanced stage ofthe production so that a hollow cavity structure is formed. Theformation of such a cavity structure may be, for example, needed for thefabrication of a silicon microphone, wherein the cavity structuredefines a hollow space between two membrane layers so that a deflectablemembrane layer of the silicon microphone can be deflected in dependenceof a detected acoustic wave. Such a silicon microphone may be formed asa condenser microphone.

SUMMARY OF THE INVENTION

Some embodiments relate to a method for fabricating a cavity structureusing a carbon layer as a sacrificial layer, wherein the carbon layer isremoved by means of dry etching so that a cavity structure is formed.Further embodiments relate to a method for fabricating a semiconductorstructure which comprises a cavity structure, and wherein carbon is usedas a sacrificial material in order to form the cavity structure. Furtherembodiments relate to a method for forming a cavity structure for asemiconductor device using a carbon layer which is deposited by chemicalvapor deposition (CVD), and wherein the carbon layer is removed by dryetching so that a cavity structure between overlapping parts of a firstand a second layer is formed. In further embodiments, a method forforming a cavity structure for a semiconductor microphone is describedand a semiconductor microphone which is fabricated using the method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic side view of a cavity structure to describe amethod for fabricating a cavity structure according to an embodiment;

FIG. 2a shows a schematic side view of a cavity structure to describethe fabrication of a cavity structure using an opening in a first orsecond layer;

FIG. 2b shows a schematic side view of a cavity structure comprising arecess in a first layer and using an opening to remove a sacrificialcarbon layer so that the cavity structure is formed;

FIG. 2c shows a schematic side view of a cavity structure comprising aninterfacial layer between the cavity structure and a first layer;

FIG. 2d shows a schematic side view of a cavity structure using aninterfacial layer between the sacrificial carbon layer and a secondlayer;

FIG. 2e shows a schematic drawing of a cavity structure built by anoverlapping region of a first layer and a second layer after removing anintermediate sacrificial carbon layer;

FIG. 2f shows a schematic drawing of a cavity structure formed in arecess structure wherein the cavity structure is defined by anoverlapping region of a first and a second layer;

FIG. 2g shows a schematic drawing of a round cavity structure defined byan overlapping region of a first and a second layer;

FIG. 3a shows a schematic side view of semiconductor microphonestructure to describe the formation of a cavity structure for thesemiconductor microphone, wherein carbon is used as a sacrificial layer;

FIG. 3b shows a schematic side view of semiconductor microphonestructure to describe the formation of the cavity structure, wherein thesacrificial carbon layer is covered by a nitride layer and encapsulatedby an oxide layer;

FIG. 3c shows a schematic side view of depositing a second membranelayer as a lid on top of a carbon layer covered with the nitrite oxidelayers;

FIG. 3d shows a schematic silicon microphone after removing the carbonlayer so that a cavity structure between the first and second membranelayers is formed;

FIG. 4a shows a side view of a silicon microphone comprising a carbonsacrificial layer between a first and second membrane layer according toanother embodiment; and

FIG. 4b shows a schematic side view of a silicon microphone comprising acavity structure between a first and a second membrane layer afterremoving the carbon sacrificial layer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In embodiments, a method for fabricating a cavity structure, forexample, a cavity structure for a semiconductor structure or device isdisclosed. In addition, embodiments for a method for fabricating asemiconductor structure, for example, a semiconductor microphone aredescribed.

The fabrication of such a cavity structure may be used for thefabrication of micro-electromechanical structures (MEMS). Such aMEMS-structure can be, for example, a sensor or microphone which is ableto detect a change of a pressure. The sensor or microphone may beconfigured as a pressure sensitive electrical capacitor. The pressuresensitive sensor can comprise two electrodes separated by a defineddistance—an “air gap” with a defined thickness—which forms a dielectricmedium. The air gap may be formed by a cavity structure whosefabrication is described herein. At least one of the two electrodes ormembranes, are movable or deflectable. In dependence of the appliedpressure, the thickness of the gap or the distance between the twoelectrodes is variable, and hence, the detectable capacitance valuesdepend on the applied pressure.

Usually, for the fabrication of such a semiconductor structure, e.g. asilicon microphone, Tetra-Ethyl-Ortho-Silicate (TEOS) oxide is used todefine the thickness of the gap or the cavity structure. The TEOS-oxidelayer may act as a spacer and may be removed at a later stage of thefabrication process. The removal of the gap defining TEOS-oxide layer isusually performed by wet chemical etching, wherein the exchange of thefluid media, the etching medium and the reaction products between thegap region and the environment has to be performed through smallopenings in the electrodes. This necessary exchange of the fluid mediais normally a critical step because of the requirement of defect freeand residue-free removal of the TEOS-oxide layer in the gap or cavitystructure region. Openings in the electrodes should be clean andresidue-free and the membranes or electrode layers should be easilymovable or deflectable without any sticking. After the removal of theTEOS oxide layer by means of wet chemical etching a subsequent wetchemical cleaning and drying step is performed. These cleaning anddrying steps can be also critical because of a possible sticking of thetwo membrane layers. This is usually avoided by performing hydrophobiccleaning methods which result in clean surfaces and therewith free andeasily movable membranes. But applying wet chemical etching to achieve aclean cavity structure requires additional expensive and time consumingprocess steps.

If a wet chemical etching is performed an unwanted underetching of thecavity structure and a contamination with impurities may arise. Thismeans, applying a wet chemical etching in order to remove the auxiliarymaterial, e.g. silicon oxide and to form the cavity structure has somedisadvantages. For the wet chemical etching of silicon oxidehydrofluoric acid (HF) which is selective to silicon, may be used asetching medium. But nevertheless, a strong underetching at the sidewalls of the membrane and the creation of impurities remain asdisadvantages. In addition, the time for performing the wet chemicaletching is relatively long and the necessary equipment quite expensive.A further disadvantage is the usage of chemicals which are dangerous forthe environment and human beings. To minimize the risk of anenvironmental pollution, expensive equipment has to be purchased andwaste disposal measures have to be implemented.

A silicon oxide layer which may be deposited by CVD or which may be athermal grown oxide can be used as material to define the gap betweenthe two membrane layers. The removal of the silicon oxide layer can beperformed by applying a gentle reaction of the latter SiO₂ layer, with agaseous HF, e.g. HF-vapor.SiO₂+4HF→SiF_(4(gas))+2H₂O(H₂O has to be removed—otherwise hydrolysis)SiF₄+2H₂O→SiO_(2(aq))+4HFSiO_(2(solid))+6HF_((gas))→H₂SiF₆+2H₂0

However, as can be seen in the formula, water (H₂0) is a reactionproduct which has to be removed from the cavity structure.

In embodiments for the fabrication of a cavity structure, movablemembranes, for example, for a silicon microphone, can be embedded inother auxiliary or sacrificial materials. According to embodiments,carbon is used as auxiliary material or sacrificial layer which is thenremoved in a later stage of the fabrication process. The carbon layermay be deposited using a chemical vapor deposition (CVD) technique andcan be easily and inexpensively removed, for example, by applying anoxygen plasma in a dry etching process. According to some embodiments,the removal of the carbon layer may be performed during the removal ofthe photo resist (lack stripping), so that a minimization of the processtime can be achieved. This means, during the removal of the photo resistalso the carbon layer can be removed by the applied oxygen plasma.

In further embodiments another etching medium instead of oxygen may beused for dry etching and removing the carbon layer. The respectiveetching medium for the dry etching should have a high selectivity to themembrane layer and also to a possible support structure or interfaciallayer for the membrane layer. According to some embodiments the carbonlayer may be removed by means of an isotropic dry etching process.

During fabrication the sacrificial carbon layer may be encapsulated sothat it is not attacked or destroyed by a subsequent unwanted processstep. This can be achieved by depositing a second membrane layer or aninterfacial layer on top of the carbon layer in the shape of a lid. As aresult, circumferential side walls of the carbon sacrificial layer maybe also covered and protected against an unwanted influence during thefabrication process. The second membrane layer may have an opening oropenings may be formed or etched in the second membrane layer, so thatthe sacrificial layer can be attacked and removed by a dry etchingmedium. The carbon layer can be removed residue-free in a gaseous phasesolid state reaction. According to further embodiments, the carbon layermay be protected at the side wall by a silicon oxide. In this case, thesecond membrane layer may be flat or planar and may not comprise theabove mentioned shape of a lid.

The carbon layer may, for example, be deposited by CVD. The depositedcarbon layer may comprise a sufficient mechanical stability fordepositing in a subsequent process the second membrane layer. Thesacrificial carbon layer can be removed by a gentle reaction of thecarbon layer with gaseous oxygen (O₂), for example, by a thermaloxidation or plasma oxidation. Thereby, the following chemical reactionmay take place:2C_((solid))+O_(2(gas))→2CO_((gas))2CO_((gas))+O2_((gas))→2CO_(2(gas))

The reaction products, CO and CO₂ can easily be removed since they arealso gaseous. Therewith, the sticking problem may be avoided, which mayarise from the usually applied wet chemical etching and the subsequentcleaning and drying steps.

According to embodiments, a sacrificial layer made of carbon can beremoved by applying one or a sequence of gently gaseous phase solidstate reaction instead of using process critical wet chemical etchingand cleaning processes. Therefore, a critical fluid medium exchange,i.e. an exchange of an etching medium and etching reaction products,through openings of a membrane layer can be avoided. Furthermore, nowetting weaknesses of the materials forming the gap or cavity structuremay occur. A removal of the sacrificial layer by means of a gaseousphase solid state reaction allows a good exchange of the gaseous etchmedium and the gaseous reaction products through openings in themembrane.

No critical drying step after the etching of the gap or cavity structureis necessary, since dry etching is performed. The exchange of the gasescan be performed directly in the gas phase reaction chamber withoutmechanically moving the wafer or the substrate. The described method forfabricating a cavity structure can result in a reduced defect density,since a reduced mechanical movement and handling of the wafer andetching medium is necessary compared to a method using wet chemicaletching. In contrast, if a wet chemical etching is performed a wafer orsubstrate has to be carried between the etch cell to a rinsing stationand further on to a drying station, so that the wafer or substrate hasto be transported frequently which may result in damage and therefore ina higher defect density.

By dry etching the pure carbon can be chemically converted to carbonmonoxide (CO), respectively to carbon dioxide (CO₂) wherein both gasesare dry gases that can be removed residue-free. Therewith, a sticking, acontamination with left impurities and an environment pollution can beavoided. Furthermore, the described method for fabricating a cavitystructure, e.g. for a semiconductor structure can be performed using abatch process so that a plurality of wafers or substrates can beprocessed at the same time. In contrast, the wet etching process may beperformed in a single wafer process, so that the process time issignificantly longer and the production more expensive.

The usage of carbon as a sacrificial layer for forming gaps or cavitystructures may comprise a couple of advantages. Compared to the usualmethod for forming a cavity structure using a sacrificial oxide layer—,for example, a TEOS layer,—the process costs can be reduced. The carbonlayer can be removed residue-free by dry etching, since no or almost noparticles or residuals of the gaseous etching medium remain in thecavity structure, since the carbon layer can be completely dissolved inthe gaseous reaction products.

According to another aspect, no special and expensive equipment has tobe used to perform the method as described in the embodiments herein. Noadditional mask step for evaluation and therefore no additional costsmay be necessary. The non-uniformity of the carbon deposition is lessthan 10%. To protect the deposited carbon layer, it may be encapsulatedwith a nitride and/or oxide interfacial layer or by means of the secondmembrane layer. In addition, a wet resist strip after structuring of thecarbon can be performed.

In the following, embodiments will be described in more detail by meansof a plurality of FIGS. 1 to 4 b.

As it is schematically depicted in FIG. 1, according to embodiments, themethod for fabricating a cavity structure may be performed by providing10 a first layer 11, depositing 20 a carbon layer 21 on the first layer11, covering 30 at least partially the carbon layer 21 with a secondlayer 31 to define the cavity structure 41 and removing 40 by a means ofdry etching the carbon layer 21 between the first 11 and second layer 31so that the cavity structure 41 is formed.

The first layer 11 may be, for example, a semiconductor substrate, aninsulating layer, a conductive layer, a metal layer, a layer made of aninorganic or organic material. The first layer may be a layer made ofglass, ceramic, plastic or other synthetic material. In some embodimentsat least one of the first and second layer 31 may comprise polysilicon.The polysilicon may comprise p- or n-type dopants. The first layer 11may be a semiconductor wafer. According to some embodiments, the firstlayer may comprise a thickness between about 1 nm to about 1 cm, betweenabout 10 nm and about 100 μm or between about 10 nm and about 10 μm. Thefirst layer may be configured as a membrane layer.

As it is shown in FIG. 1, a carbon layer 21 may be deposited on thefirst layer 11. The deposition may take place on a first main surface 11a of the first layer which is opposite to a second main surface 11 b ofthe first layer 11. The carbon layer 21 may be in direct contact to thefirst layer 11. According to some other embodiments, at least oneinterfacial layer may be arranged between the carbon layer 21 and thefirst layer 11.

According to some embodiments, the carbon layer 21 may be depositedusing chemical vapor deposition (CVD). The carbon layer 21 may bedeposited using a low pressure chemical vapor deposition (LPCVD) method.According to some embodiments, the deposition of the carbon layer may beperformed in a temperature range between about 600° C. to about 1000°C., between about 700° C. and about 900° C., or between about 750° C. toabout 850° C. The deposition of the carbon layer may be, for example,performed at a pressure between about 1 Torr to about 700 Torr, about 1Torr to about 500 Torr, or between about 90 Torr to about 110 Torr, e.g.about 100 Torr. The carbon may be deposited in a batch process, e.g. ina batch oven process. This means that the deposition of the carbon layercan be performed for a plurality of cavity structures at the same time.For the deposition of the carbon layer, a precursor can be used, forexample, Ethane or Acetylene. In general, there may be a plurality ofgases which comprise and which may be used as a precursor for thedeposition of the carbon layer. In some embodiments the temperature forthe deposition of carbon may be between about 700° C. and about 900° C.In case Ethane is used as a precursor, the carbon deposition temperaturemay be between about 830° C. and about 870° C. It may be, for example,about 850° C. In case Acetylene is used as precursor, the carbondeposition temperature may be between about 730° C. and about 770° C. Itmay be, for example, about 750° C.

According to some embodiments, the carbon precursor gas may be dilutedwith nitrogen gas (N₂) during the deposition of carbon. The overall gasflow during the deposition of the carbon layer by CVD may be betweenabout 0.5 slm to about 8 slm, or for example, between about 1 slm toabout 5 slm. The carbon precursor share may be in a range of about 10%to about 90% or about 20% to about 80% of the overall gas flow.According to some embodiments, a total pressure or pressure may be in arange of about 1 Torr to about 500 Torr, for example, about 100 Torr,during the deposition of the carbon layer.

The deposition of the carbon layer may be performed so that the carbonlayer comprises a graphite structure in a share of about 20% to about90% of the carbon layer. The carbon deposition can be performed so thatthe non-uniformity of the deposited carbon layer is lower than about10%. This means the carbon layer can be deposited with a highuniformity.

As it is schematically shown in FIG. 1, the carbon layer 21 may becovered in a subsequent covering process 30 at least partially with asecond layer 31. The cavity structure 41 may be defined by covering ordepositing the second layer on the carbon layer. According to someembodiments, the cavity structure 41 is defined by an overlapping partof the second layer 31, the carbon layer and the first layer 11. Thismeans, according to some embodiments the cavity structure may be justdefined by the overlapping part of the first and second layer. A sidewall which may encapsulate the cavity structure is not needed to definethe cavity structure. According to other embodiments the cavitystructure is defined by the overlapping part of the first and secondlayers and by at least one side wall. According to some embodiments, thecarbon layer may be completely covered by the second layer. This means,also the side walls of the carbon layer 21 may be completely or at leastpartially covered with a second layer. Covering 30 the carbon layer withthe second layer 31 may be performed so that a main surface 21 a of thecarbon layer and a side wall 21 b of the carbon layer is covered atleast partially by the second layer 31.

The second layer 31 may be made of a semiconductor material, aninsulating material, or a conductive material. The second layer 31 maybe configured as a membrane layer. The second layer may be a metalliclayer or a semiconducting layer, for example, made of polysilicon whichcan be doped with n- or p-type dopants. The second layer may be an oxideor nitride layer. It may comprise titanium, wolfram, aluminum, copper,silicon, silicon oxide, gallium arsenide, indium phosphide, organic-,polymeric materials or other materials which may be used, for example,in the semiconductor process technology.

As it is schematically depicted in FIG. 1, the cavity structure isformed by removing 40 the carbon layer between the first and secondlayers by a means of dry etching. The carbon layer can be removed bymeans of dry etching using oxygen (O₂), for example, during thermaloxidation or plasma oxidation. The sacrificial carbon layer can beremoved by means of different dry etching methods, so that no wetetching process is necessary in order to form the cavity structure 41.Dry etching methods may involve, for example, deep reactive ion etching(DRIE) methods, reactive sputtering, Bosch etching methods, reactive ionbeam etching techniques, barrel etching techniques, sputter etchingtechnique or ion beam etching techniques. The carbon layer 21 may beremoved using plasma or a barrel dry etching technique. The dry etchingprocess may be performed as a batch process so that, for example, aplurality of wafer can be etched at the same time. As a consequence, ahigh throughput of wafers can be achieved.

The dry etching process is performed using an etching medium with a highetch selectivity with respect to the carbon layer and the surroundingfirst-, second-, or interfacial layers, so that only or mainly thecarbon layer is removed and the surrounding layers are not, or onlymarginally attacked by the dry etching medium. By means of the dryetching, a complete removal of the sacrificial carbon layer in thecavity structure between the first and the second layers can beachieved, so that the cavity structure or under etching is achieved.According to some embodiments the dry etching may be performed as anisotropic etching process. The carbon layer 21 may be reduced to ashesby the dry etching process so that a clean and smooth cavity structurewithout residuals of carbon can be formed. This means, the dry etchingprocess of the sacrificial carbon layer, e.g. by an oxygen plasma,permits in a simple manner the fabrication of a clean and residue-freecavity structure.

According to embodiments for fabricating a cavity structure, carbon canbe used as sacrificial layer, wherein the carbon layer is arrangedbetween a first and a second layer whose overlapping area defines thecavity structure. The sacrificial carbon layer may be removed by meansof dry etching, so that a clean and residue-free cavity structure isformed. Such a cavity structure may be used, e.g. in different types ofmicro-sensors, micro-mechanical devices, micro-electromechanical devicesor micro-optical devices.

In FIG. 2a , a schematic side view of an interstage cavity structure isdepicted. On top of a first main surface 11 a of a first layer 11, acarbon layer 21 is deposited as described above. Afterwards the carbonlayer 21 is completely covered by the second layer 31. This means, thatthe main surface 21 a, as well as the side walls 21 b of the carbonlayer 21 are covered by the second layer 31. The carbon layer may becompletely encapsulated by the second layer 31 and the first layer 11.According to some embodiments, the method for fabricating a cavitystructure may further comprise the formation 50 of at least an opening51, in at least one of the first 11 or second layer 31. The opening 51may used to allow an exchange of the etching medium and the reactionproducts during the subsequent dry etching 40 of the carbon layer 21.This means, according to some embodiments at least one opening 51 isformed in the second layer and/or in the first layer 11 (shown as dashedline), so that a dry etching medium can attack the encapsulated carbonlayer 21 during the removal 40 of the carbon. It should be noted thatthe first layer may already comprise an opening 51 when it is provided10 to deposit the carbon layer on a main surface 11 a of the first layer11. According to other embodiments, an opening 51 in the first layer orsecond layer may be formed in a separate step 50. An opening 51 in thesecond layer can be generated during the process of covering 30 thecarbon layer 21 with the second layer 31, or it may be formed in asubsequent step 50. The opening 51 may be suitable for a dry etchingmedium to attack and remove the sacrificial carbon layer 21.

As it is schematically shown in FIG. 2b , the first layer 11 maycomprise a recess or trench structure 11 c. This means, providing 10 afirst layer 11 may be performed so that the first layer 11 comprises arecess or trench 11 c. The deposition 20 of a carbon layer 21 isperformed so that the recess or trench structure 11 c is filled with thesacrificial carbon layer 21. According to this embodiment the coverage30 of the carbon layer may be performed so that the carbon layer 21 andthe recess 11 c is at least partially covered with the second layer 31.In FIG. 2b , the carbon layer 21 is completely covered with the secondlayer 31. In this case, at least one opening 51 may be formed 50 in thesecond layer 31, so that by means of dry etching the carbon layer 21 canbe removed 40 and the cavity structure 41 built.

According to another embodiment (see FIG. 2c ), a first layer 11 may beprovided and on top of the first main surface 11 a of the first layer 11an interfacial 61 may be deposited. Afterwards, a carbon layer 21 may bedeposited 20 on top of this first layer and the interfacial layer 61, asdescribed above. This means, between the carbon layer 21 and the firstlayer 11 at least one interfacial layer 61—the first layer interfaciallayer—is arranged. Then, the carbon layer 21 is covered, at leastpartially, as described above with the second layer 31. According tothis embodiment, the second layer 31 and the first layer 11 are not indirect contact to each other. Both are separated by the interfaciallayer 61. The first layer interfacial layer 61 may be, for example, aninsulating layer and the first and second layers 11 and 31 may besemiconducting or conductive layers which are electrically insulatedagainst each other by the interfacial layer 61. The first layer and theinterfacial layer between the carbon layer and the first layer mayphysically separate the first and the second layers from each other. Atleast an opening in the second layer 31 and/or an opening in the firstlayer 11 and the interfacial layer 61 may be formed so that in asubsequent dry etching process the sacrificial carbon layer 21 can beremoved. According to some embodiments, the first layer interfaciallayer 61 may, for example, be an insulating layer made of nitride oroxide. In further embodiments the first layer interfacial layer 61 maycomprise two or more different layers.

As it is schematically shown in FIG. 2d , according to otherembodiments, the carbon layer 21 may be covered by a second layerinterfacial layer 71. The second layer interfacial layer 71 may preventa direct contact of the second layer 31 and the carbon layer 21. Thesecond layer interfacial layer 71 may be made of nitride or oxide.According to other embodiments, the second layer interfacial layer 71may comprise a plurality of different layers arranged between the secondlayer 31 and the carbon layer 21. The second layer interfacial layer 71may comprise a nitride/oxide layer which may mechanically support andstabilize the second layer 31. The second layer 31 may be formed as amembrane layer.

At least one opening 51 may be formed in the second layer 31 and thesecond layer interfacial layer 71 so that a dry etching medium canattack the carbon layer 21 and to remove the encapsulated carbon layer21, so that the cavity structure 41 is formed. According to otherembodiments, a cavity structure 41 may comprise a first layerinterfacial layer 61, as described in the context of FIG. 2c , and asecond layer interfacial layer 71, as described in context of FIG. 2 d.

In FIG. 2e , a schematic perspective view of a cavity structurefabricated with the inventive method is depicted. The cavity structure41 which is defined by an overlapping part of the second layer 31 with afirst layer 11 is schematically depicted by the dashed line. Accordingto embodiments, a cavity structure 41 may comprise at least one sidewall. In further embodiments, a cavity structure 41 may be defined by anoverlapping, under etched region of the first layer and the secondlayer.

In FIG. 2f , a perspective view of another cavity structure 41 isdepicted. In this embodiment, a carbon layer has been deposited in arecess structure 11 c of the first layer 11, as described in context ofFIG. 2b . After depositing 20 the carbon layer in the recess structure11 c, the carbon layer is covered 30, at least partially, with thesecond layer 31. After removing 40 the carbon layer by a means of dryetching, the cavity structure 41 with the freestanding cantileverstructure 31 d is formed. In this embodiment, the cavity structure 41 isdefined by the overlapping parts of the second layer 31, the removedcarbon layer and the first layer 11.

In FIG. 2g , a schematic perspective view of a round cavity structure 41is depicted. In this embodiment, the carbon layer has been deposited 20in a round shape on top of a main surface 11 a of a first layer 11.Afterwards, the carbon layer has been completely covered with around-shaped second layer 31 so that a kind of disc or lid is formed.The lid covers the main surface of the carbon layer 21, as well as, thecircumferential side wall of the carbon layer. The second layer 31 maycomprise at least one opening 51 which may be formed during thedeposition of the second layer or in a separate step so that an etchingmedium can attack the encapsulated carbon layer. The encapsulated carbonlayer is completely removed so that a round disc-like cavity structure41 is formed which is defined by the overlapping part of the secondlayer, the removed carbon layer and the first layer. It should bementioned that different types of cavity structures with differentgeometry, size and shape can be formed by means of the inventivemethods.

According to some embodiments, the fabrication of the semiconductorstructure may comprise the step of providing 10 an interstagesemiconductor structure comprising a semiconductor material and asacrificial material made of carbon. The sacrificial material is removedfrom the interstage semiconductor structure in order to form thesemiconductor structure. The removal 40 of the sacrificial layer may beperformed by means of dry etching, for example, by plasma etching or areactive ion etching. Embodiments describe a method for forming a cavitystructure for a semiconductor device wherein the method comprisesapplying a chemical vapor deposition technique to deposit a carbon layeron a first layer, wherein the chemical vapor deposition is performed ata temperature between about 700° C. and about 900° C., and at a totalpressure or pressure between about 1 Torr and about 500 Torr. The methodfurther comprises to cover, at least partially, a carbon layer with asecond layer, so that a cavity structure is defined by an overlappingpart of the second layer covering the carbon layer and the first layer.Then, the cavity structure is formed by removing the carbon layer, atleast in the overlapping part, by an etching medium etching.

In the following, an embodiment for a method for forming a cavitystructure for a semiconductor microphone is described. Such asemiconductor microphone may be realized as condenser microphonecomprising two membrane layers which form a capacitor and which areseparated by the cavity structure. Depending on an incoming sound waveat least one of the two membrane layers may be deflected or moved in thedirection of the other membrane layer, so that the capacity of thecapacitor changes. Therewith, the sound wave can be transformed in anelectrical signal.

FIG. 3a shows a schematic side view of an interstage semiconductorstructure for a semiconductor microphone. The interstage semiconductorstructure 81 may comprise a semiconductor substrate 82, wherein on a topof a first 82 a and a second 82 b main surface an insulating layer 85 isarranged. The semiconductor substrate 82 may be, for example, a siliconsubstrate, and the insulating layer 85 may be a silicon oxide layer. Aconductive first membrane layer 11 is deposited on the first mainsurface 82 a of the semiconductor substrate 82. The conductive firstmembrane layer 11 may be the first layer, as described in context of theabove mentioned FIGS. 1 to 2 g, wherein the conductive first membranelayer 11 is made of polysilicon, for example. By means of chemical vapordeposition, a carbon layer 21 is deposited 20, as described herein, onthe first membrane layer 11. The first membrane layer may be fabricatedas poly1 in a semiconductor process. The chemical vapor deposition maybe performed at a temperature between about 700° C. and about 900° C.,and at a pressure between about 1 Torr to about 500 Torr. In thisembodiment the carbon layer may also cover the oxide layer 85 at thesecond main surface 82 b of the substrate 82. According to someembodiments deepenings 23 for a subsequent formation of bumps may beetched in the deposited carbon layer 21.

In FIG. 3b on top of the carbon layer 21, a nitride 90 layer isdeposited. Therewith, the deepenings 23 are filled with nitride 90 sothat the bumps 24 are built. Outside of the membrane region and the onthe backside of the semiconductor substrate 81 b the carbon may beremoved. Afterwards, the carbon layer between the nitride layer 90 andthe first membrane layer 11 may be encapsulated by an oxide layer 95.The oxide layer 95 may cover the side walls 21 b of the carbon layer 21.The oxide layer 95 may encapsulate the carbon layer so that the carbonlayer is protected in subsequent process steps against an unwantedattack of the carbon layer. In order to form the cavity structure forthe silicon microphone, a conductive second membrane layer 31 which maycomprise a couple of openings 51, may be deposited on top of the carbonlayer. The second membrane layer 31 may comprise, e.g. polysilicon,which is n- or p-type doped. In the enlargement 99 the openings 51 andthe bumps 24 are shown more detailed.

As it is schematically shown in FIG. 3c , the second membrane layer 31may cover a main surface 21 a of the carbon layer, as well as, at leastpartially the circumferential side wall 21 b of the carbon layer.Therefore, the second membrane layer 31 comprises the shape of a lidwhich is put on the carbon layer. The lid made of the membrane layer 31may serve as a protection against unwanted subsequent process stepswhich might attack the carbon layer. Then openings 51 are etched in thesecond membrane layer 31.

In FIG. 3d , openings 51 are also etched in the nitride and oxide layers90, 95 which form the above described second layer interfacial layer 71.The etching of the interfacial layer 71 may be performed by means of areactive Bosch etching process. Afterwards, the carbon layer 21 can beremoved 40 by means of dry etching, wherein the dry etching mediumattacks the carbon layer through the openings 51 in the second membranelayer. After removal of the carbon layer, the cavity structure betweenthe first 11 and second 31 membrane layers is formed. The secondmembrane layer 31 is mechanically reinforced by the nitride, oxidelayers 90, 95. In order to manufacture a functional silicon microphonefurther process steps may be necessary. For example, a backside etchingfrom the second main surface 82 b of the substrate 82, down to the firstmembrane layer 11 may be performed and furthermore, electricalconnections and terminals to the conductive first and second membranelayers may be formed.

FIG. 3d depicts the schematic side view of a semiconductor microphone,which comprises a conductive first membrane layer 11, separated by acavity structure 41 from a conductive second membrane layer 31, whereinthe second membrane layer 31 comprises a circumferential side wall 31 bwhich is extending in the direction to the first membrane layer 11, sothat the cavity structure 41 is defined by the first membrane layer 11and the second conductive membrane 31 with the circumferential side wall31 b. The second membrane layer 31 with the circumferential side wall 31b may be formed so that during the fabrication of the semiconductormicrophone, the sacrificial layer 21 is encapsulated and protected atthe side walls 21 b. The same can be accomplished by the oxide layer 95which can in addition or alternatively cover the side walls 21 b of thecarbon layer 21. This means, the carbon layer 21 is protected against anunwanted attack of the carbon layer during the application of furtherprocess steps until the carbon layer is removed by dry etching. Thesecond membrane layer 31 may have the shape of a cover lid or lid. Thesecond membrane layer 31, as well as the first membrane layer 11 maycomprise openings 51.

FIG. 4a shows a schematic drawing of a pressure sensitive MEMS, forexample, a (DSOUND) silicon microphone before the gap defining carbonlayer 21 is removed. The etch construction 98 serves for mechanicalstabilization of the electrodes 11 and 31. The second electrode 31 maybe moveable or deflectable, if the cavity structure defining carbonlayer 21 is removed.

As it is shown in FIG. 4b , a semiconductor microphone may comprise aflat or plane second membrane layer 31 without the circumferential sidewall 31 b, as described in context of FIG. 3d . A sacrificial layer madeof carbon 21 is arranged between the first and second membrane layer.The side walls 21 b of the carbon layer may be protected by an oxidelayer 95. After the removal of the sacrificial layer the semiconductormicrophone is formed. The silicon microphone 100 may be a condensermicrophone with a first 11 and a second 31 conductive membrane layerwhich are electrically isolated from each other, and wherein eachmembrane layer comprises their own electrical connections and terminals93.

In some embodiments the second layer 31 may comprise, for example, athickness between about 10 nm and about 10 μm or between about 10 nm andabout 100 nm. The thickness of the deposited carbon layer 21 may be, forexample, between about 100 nm and about 10 μm or between about 1 μm andabout 4 μm and the thickness of the first layer 11 may be, for example,between about 10 nm and about 10 μm or between about 10 nm and about 100nm.

It should be noted, that while this invention has been described interms of several embodiments, there are alterations, permutations, andequivalents which fall in the scope of this invention. It should also benoted that there are many alternative ways of implementing the methodsand compositions of the present invention. It is therefore intended,that the following appended claims be interpreted as including all suchalterations, permutations and equivalents as fall within the true spiritand scope of the present invention.

What is claimed is:
 1. A semiconductor microphone comprising: a firstconductive membrane layer disposed in or over a substrate, the firstconductive membrane layer being a moveable membrane of the microphone; asecond conductive membrane layer disposed over the first conductivemembrane layer; a cavity structure disposed between the first conductivemembrane layer and the second conductive membrane layer, wherein thesecond conductive membrane layer is separated by the cavity structurefrom the first conductive membrane layer, wherein the second conductivemembrane layer comprises a circumferential side wall extending in adirection to the first conductive membrane layer; an encapsulant layerdisposed between the second conductive membrane layer and the firstconductive membrane layer, the encapsulant layer comprising acircumferential side wall disposed on the circumferential side wall ofthe first conductive membrane layer; and a cover layer disposed over atop surface of the cavity structure, the encapsulant layer disposedbetween the cover layer and the second conductive membrane layer.
 2. Themicrophone of claim 1, wherein the cavity structure is a recess in asubstrate comprising the first conductive membrane layer.
 3. Themicrophone of claim 1, wherein the cavity structure is a round shapedstructure over a main surface of the first conductive membrane layer. 4.The microphone of claim 1, wherein the cover layer comprises bumpsextending into the cavity structure.
 5. The microphone of claim 1,further comprising: a back side cavity disposed under the firstconductive membrane layer.
 6. The microphone of claim 1, wherein thefirst conductive membrane layer and the second conductive membrane layercomprise poly silicon.
 7. The microphone of claim 1, wherein the firstconductive membrane layer and the second conductive membrane layer areelectrically isolated from each other.
 8. An intermediate microphonestructure comprising: a first conductive membrane layer disposed over asemiconductor substrate; a carbon layer disposed over the firstconductive membrane layer and comprising a plurality of deepenings; acover layer disposed over a top surface of the carbon layer, the coverlayer filling the plurality of deepenings; an protection layer liningsidewalls of the carbon layer and disposed over the cover layer; asecond conductive membrane layer disposed over the protection layer, thesecond conductive membrane layer comprising a plurality of openings. 9.The structure of claim 8, wherein the cover layer comprises a nitridelayer, and wherein the protection layer comprises an oxide layer. 10.The structure of claim 8, wherein the carbon layer comprises a carbonmaterial deposited at a temperature between 700° C. to 900° C.
 11. Thestructure of claim 8, wherein the first conductive membrane layer andthe second conductive membrane layer comprise poly silicon.
 12. Thestructure of claim 8, wherein the plurality of openings comprise openingfor etching the carbon layer.
 13. The structure of claim 8, wherein thefirst conductive membrane layer comprises a plurality of depressionsfilled with the carbon layer.
 14. A semiconductor microphone comprising:a first conductive membrane layer comprising a moveable membrane of themicrophone; a second conductive membrane layer disposed over the firstconductive membrane layer; a cavity structure disposed between the firstconductive membrane layer and the second conductive membrane layer,wherein the second conductive membrane layer is separated by the cavitystructure from the first conductive membrane layer, wherein the secondconductive membrane layer comprises a circumferential side wallextending in a direction to the first conductive membrane layer; anencapsulant layer disposed between the second conductive membrane layerand the first conductive membrane layer, the encapsulant layercomprising a circumferential side wall disposed on the circumferentialside wall of the first conductive membrane layer; and a cover layerdisposed over a top surface of the cavity structure, the encapsulantlayer disposed between the cover layer and the second conductivemembrane layer, wherein the cover layer comprises bumps extending intothe cavity structure.
 15. The microphone of claim 14, wherein the cavitystructure is a recess in a substrate comprising the first conductivemembrane layer.
 16. The microphone of claim 14, wherein the cavitystructure is a round shaped structure over a main surface of the firstconductive membrane layer.
 17. The microphone of claim 14, furthercomprising: a back side cavity disposed under the first conductivemembrane layer.
 18. The microphone of claim 14, wherein the firstconductive membrane layer and the second conductive membrane layercomprise poly silicon.
 19. The microphone of claim 14, wherein the firstconductive membrane layer and the second conductive membrane layer areelectrically isolated from each other.